Material processing with use of an ultra-fast laser (i.e., laser output having pulse widths of less than 20 ps) or a mode-locked laser having laser output pulse widths shorter than 1 ns is currently a popular topic for discussion among industry practitioners. A typical commercially available ultra-fast laser is capable of a laser pulse repetition rate of 1-5 KHz, with 1-5 mJ laser energy per pulse. FIG. 1A shows the building block components of a typical prior art ultra-fast laser system 10. FIG. 1B shows typical laser pulse waveforms produced at the outputs of different components of system 10. The first part of ultra-fast laser system 10 is a typical mode-locked laser 12. Constructed with proper lasing materials and mode locking techniques, mode-locked laser 12 emits laser output pulse power characterized by a pulse width of 100 fs-20 ps and pulse repetition rate of 80-100 MHz. A laser pulse picking device 14 selects 1-5 KHz mode-locked laser pulses from the 80-100 MHz mode-locked laser pulse train for amplification. FIG. 1B, lines A and B, show the pulse trains at the outputs of mode-locked laser 12 and pulse picking device 14, respectively. Risk of damage to any optical components by the amplified intense ultra-fast laser pulses is avoided by introducing at the output of pulse picking device 14 a pulse stretcher 16 that stretches the femtosecond-wide laser pulses to reduce their peak intensity before delivery to a regenerative amplifier 18. After amplification, the amplified stretched pulses are directed to a pulse compressor 20, which restores them to the desired femtosecond pulse width range. FIG. 1B, line C, shows the amplified output of pulse compressor 20. Laser system 10 is very complex, very expensive, and difficult to use in an industry environment. Moreover, the laser pulse repetition rate is too low for many laser processing applications.
On the other hand, a typical mode-locked laser has a laser pulse repetition rate of 80-100 MHz (depending on the resonator length) with a relatively very low laser energy per pulse (in the range of less than 1 μJ). FIG. 2A shows a prior art mode-locked laser system 30 composed of a mode-locked laser 32 emitting laser output that has a pulse width in a range of between one picosecond and several tens of picoseconds and is directed to an optional pulse picking device 34, which is followed by an optional amplifier 36. FIG. 2B, lines A, B, and C, show typical laser pulse power waveforms produced at the outputs of mode-locked laser 32, pulse picking device 34, and amplifier 36, respectively. Amode-locked laser system 30 has recently become available, such as a Time-Bandwidth Products, Inc. Duetto model laser, and exhibits typical laser repetition rate of 100 KHz at an average power of about 10 W. There are several performance, packaging, and operational problems associated with this kind of laser system. First, since the pulse repetition rate of mode-locked laser 32 is as high as 100 MHz but the pulse repetition rate of pulse picking device 34 is only 100 KHz, most (99.9 percent) of the laser energy emitted by mode-locked laser 32 is wasted. The laser energy per pulse for mode-locked laser 32 is too low, which places very stringent gain requirements on amplifier 36. Second, the design and structure of laser system 30 are very complex. Third, because of such complexity, laser system 30 is not yet ready for widespread industrial application.
A simultaneously mode-locked, Q-switched laser system is constructed with a Q-switch placed in the resonator of a mode-locked laser to control its laser pulse repetition rate. The laser output pulse emitted from the mode-locked laser is a higher laser energy pulse because of Q-switch operation. FIG. 3A shows a prior art simultaneously mode-locked, Q-switched laser system 40 composed of a Q-switched, mode-locked laser 42, together with an optional pulse picking device 44, and an amplifier 46. FIG. 3B, lines A, B, and C, show typical laser pulse power waveforms produced at the outputs of mode-locked, Q-switched laser 42, pulse picking device 44, and amplifier 46, respectively. FIG. 3B, line C, shows the output pulse waveform of laser system 40, which waveform is characterized by multiple mode-locked laser pulses 48 under a nanosecond-wide Q-switched laser pulse power profile 50. Pulse power profile 50 of laser pulses 48 is advantageous for many laser processes, such as, for example, semiconductor memory link processing, material trimming, and via formation. The number of mode-locked laser pulses 48 within the nanosecond-wide pulse power profile 50 can be controlled by the Q-switching operation or selected with an optional pulse picking device 44 for best processing results. The short time interval between next adjacent ones of multiple mode-locked pulses 48 is advantageous for many laser material processes for reducing debris and increasing throughput. The laser energy of each mode-locked laser pulse 48 can be increased by use of an optional amplifier 46. One potential technical difficulty for this Q-switched and mode-locked laser is that mode locking cannot be ideally established during the short time of Q-switched laser pulse buildup. This is the reason why mode-locked lasers are mostly continuous-wave pumped and operated in a continuous mode, i.e., to provide sufficient time to establish mode locking or to not interrupt mode locking during operation.